EP-A-0 060 234 and EP-A-0 253 59 each disclose a measuring feeder whose storage drum consists of a plurality of circumferentially separated segments. Some of the segments can be adjusted radially asymmetrically relative to the storage drum axis to vary the circumferential length of the storage drum. The stop device contains only a single stop element which can be disengaged to release the thread for withdrawal and can be engaged again shortly before the predetermined thread length is reached in order to dimension the thread length. In the measuring feeder according to EP-A-0 253 359, an actuator is provided for adjusting the segments also during operation so as to adjust or readjust the correct thread length.
EP-A-0 253 359 describes a measuring feeder whose storage drum has an invariable diameter and a fixed circumferential length. The stop device contains a plurality of stop elements which are distributed circumferentially over the storage drum and which can individually be actuated via a control device to compose the predetermined thread length of whole turns or of fractions thereof which can be set by the circumferential distances of the stop elements.
In storage drums of an invariable diameter and in storage drums of a variable diameter, the thread length is set such that attention is paid to the reduction which is due to the thread tension stored in the elastic turns upon relaxation of the withdrawn thread section. In weft-thread measuring 3 feeders of a storage drum of the two above-mentioned types, a high speed level and/or a high thread tension will be observed during weft mixing weaving when in case of a defective feeder (thread breakage) another feeder assumes the function of the defective feeder. In each type of storage drum, a high speed level can also arise with drastic pattern variations during pattern weaving for a short time. Furthermore, an influence of the diameter of the supply bobbin on the withdrawal tension of the thread from the bobbin (or the other words, the winding tension of the thread onto the storage drum) can always be felt, i.e., the thread tension increases with a decreasing bobbin diameter. The higher the speed level and/or the stronger the resistance to withdrawal from the supply bobbin in comparison with normal operation, the stronger becomes the tension in the turns in the thread stock and the stronger is the reduction in the withdrawn relaxed thread section. That is why the thread length is set with a view to the strongest reduction to be expected to avoid short picks that lead to expensive standstill periods of the machines. This, however, means a great amount of waste caused by long free thread ends to be cut off during a considerable portion of the operation. In a weft-thread measuring feeder comprising a storage drum of an invariable diameter and a plurality of stop elements, the thread length can only be adjusted in steps which depend on the pitches of the stop elements. The pitches are limited for constructional reasons and for reasons of costs. In some cases the set thread length is therefore longer than the thread length actually required, which means an expensive waste of thread material.
It is the object of the present invention to provide a weft-thread measuring feeder either of the variable diameter type or the invariable diameter type which is of a simple construction and functions in a reliable manner and in which independently of the type of storage drum the thread length can be adjusted in an optimally short and accurate manner with a view to as little waste of thread material as possible and without the risk of short picks.